I second the recommendation for Asahidake. I can't speak to the skiing - though I've heard great things about it - but the area itself is one of the most beautiful areas in Hokkaido. It's a bit of a hike, but it's not at all hard to do via train and bus. I speak Japanese, but the whole way along, there are volunteers and staff that do speak English, and are quite friendly. They helped out with getting me to the right bus, and I'm sure they'd be happy to help you, too. And the ticket machines are multi-lingual, and aren't hard to figure out.
If you do go there, might I recommend a place to stay? The Daisetsuzan Shirakaba-sou Youth Hostel is wonderful. Don't be fooled by the name, only part of it is a youth hostel, the rest is a ryokan (Japanse inn/Bed and Breakfast). The meals a really good, their hot springs are fantastic, the staff is helpful and friendly, and the general atmosphere is warm and cozy. I'd stay there again in a heartbeat. Last time I was there, there was a woman on staff who spoke English, so you can make reservations in English with little problems. No, I don't work for them, I just have had really great stays there.